1. Field of the Invention
The present invention relates to a photoelectric conversion device used for a scanner, a video camera, a digital still camera etc.
2. Related Background Art
In recent years, a photoelectric conversion device called a CMOS sensor provided using a CMOS process has been a focus of attention. Use of the CMOS sensor particularly in a field of portable information devices is expected because of its easy loading together with peripheral circuits, low voltage driving etc. FIG. 8 shows an equivalent circuit of a photoelectric conversion device according to a conventional art. In the drawing, pixels are arranged in 2×2 array, but the pixels are not particularly limited to this array. In FIG. 8, a unit pixel includes a photodiode 1 which is a photoelectric conversion element, an amplification metal oxide silicon field effect transistor (MOSFET) 2 for amplifying a signal generated at the photodiode 1, a reset switch 4 for resetting an input of the amplification MOSFET 2 to predetermined voltage, and a select switch 5 for controlling electrical connection between a source electrode of the amplification MOSFET 2 and a vertical output line 7. Additionally, a transfer switch 3 is installed to control electrical connection between the photodiode 1 and a gate electrode of the amplification MOSFET 2.
An operation of the photoelectric conversion device will be described by using a timing chart of FIG. 9. When a vertical scanning circuit 6 selects a given line (referred to as n line hereinafter), first, a reset signal φRES(n) becomes low to turn OFF the reset switch. Then, a select signal φSEL(n) becomes high to turn ON the select switch 5, whereby a source of the amplification MOSFET 2 is electrically connected to the vertical output line 7, a source follower circuit is constituted of a selected pixel and a constant current load 9, and an output corresponding to a pixel reset state appears on the vertical output line 7. The minute that a clamp pulse φCLP becomes low to turn OFF a switch 13, a potential of the vertical output line is clamped by a clamp capacity (C0) 12. Subsequently, a transfer signal φTX becomes high for a fixed period, photoelectric charge is transferred from the photodiode 1 to a gate of the amplification MOSFET 2, and the potential of the vertical output line 7 is changed in accordance with the amount of photoelectric charge. This change is accompanied by a change of a potential of a line holding capacity (CT) 10 from an initial potential VCLP, the amount of which becomes a gain ratio of C0/(C0+CT). The potential change at the line holding capacity CT is established at a point of time when a φCT becomes low to turn OFF a switch 8. Then, signals held by the line holding capacity 10 are sequentially read out to a horizontal output line 15 by horizontal scanning pulses φH1, H2 generated from a horizontal scanning circuit 11. Accordingly, output signals of one line are outputted through an output amplifier 16.
However, in the aforementioned photoelectric conversion device of the conventional art, there are problems described below. Since a number of logic gates are included in the horizontal scanning circuit 11, the minute that a horizontal transfer pulse φH is generated, the logic gates are operated all at once so that through-current flows between a power source VDD and a ground potential GND. This through-current reduces voltage on a power supply path or a ground potential supply path to cause fluctuation in a high level and a low level of the pulse φH, consequently generating noise. This noise is superposed on an optical response signal by capacity coupling between a signal path from the line holding capacity 10 to the horizontal output line 15 and a gate electrode of a horizontal transfer gate 14 to cause S/N deterioration. If events occur in a logic circuit of a sensor peripheral circuit not only at the time of horizontal transfer pulse generation but also at other time, voltages are similarly reduced on the power supply path and the ground potential supply path. The reduced voltages are mixed into the optical response signal by capacity coupling between a group of switches disposed on the signal path, and an analog signal path. Such mixing-in of the logic circuit noise results in greatly-limited S/N characteristics in the conventional photoelectric conversion device.